1. Field of Invention
The present invention relates to a reflective liquid crystal display device.
2. Description of the Related Art
In recent years, active matrix type liquid crystal display devices have been developed and provided. The active matrix type liquid crystal display comprises electrodes on the substrates divided into individual pixels in which voltage is supplied to these pixel electrodes by providing switching devices such as thin-film transistors (TFTs) for each pixel
Such liquid crystal display devices include called twisted nematic liquid display devices (TNLCD), surface stabilized ferroelecrics liquid crystal display devices (SSFLCD), and anti-ferroelectrics liquid crystal display devices (AFLCD) in which changeover can be effected between a light condition and a dark condition by using a polarizing plate, by utilizing the change of optical properties of the liquid crystal material. However, since a polarizing plate is an essential constituent of a liquid crystal display device hang such a display mode, its optical utilization efficiency is at most about 50%.
George H Heilmeier, and Joel E Goldmacher (Appl Phys. Lett. 13 (1968)have proposed a display mode called a selective reflective mode, using a cholesteric liquid crystal material, in which light having a specific wavelength is selectively reflected (Bragg reflection) by the complex twisted structure of the liquid crystal material. In this mode, optical switches by changing a transparent condition to a dispersion (or weak dispersion) condition. The transparent condition appear because reflection of optical wavelengths other than in the visible light region is produced in a planar structural condition. The planar structural condition is that the helical axis is aligned practically perpendicularly to the device substrate faces. Then the dispersion (or weak dispersion) condition produced in a focal conic structural condition. The focal conic structural condition is that the helical axis is practically parallel to the substrate surfaces.
This mode makes possible display of comparatively high contrast, since display is performed by means of a viable light region transparent condition in planar condition and by means of a weakly dispersive transmitting condition in focal conic condition. However, regarding the reflection efficiency with the planar structure, since only a specific wavelength region of clockwise or anti-clockwise circular wave is reflected, the reflection rate has a limiting value of 50%.
One system for getting the reflectivity above 50% in this mode is described in Early Japanese Patent Publication No. H7-287214. In this system, a clockwise rotatory polymeric dispersion type cholesteric liquid crystal (PDCLC) layer that selectively reflects only clockwise circular wave and a anti-clockwise rotatory PDCLC layer that selectively reflects only anti-clockwise circular wave are clamped between a set of substrates formed with transparent electrodes. With this system, reflectivity of substantially better than 50% might be expected since both anti-clockwise and clockwise circular wave can be reflected. However, because of the polymeric dispersion type structure, it is difficult to control precisely the matrix condition in order to form a twisted structure. Also, the drive voltage is raised by the formation of a mixed condition with the polymer. In particular, there is the problem that, in the case of a material of high threshold value, due to the twisted structure of the liquid crystal material, an extremely large voltage is required for driving purposes.
A further problem with this system is that contrast can't be increased, since light is dispersed when the light is transmitted. In addition to the comparatively high drive voltage is required.